Natural gas is being ever more important as a source of fuel for both building heating as well as a transportation fuel. Natural gas is comprised primarily of methane with up to about 20 wt % heavier (C2+) hydrocarbons, primarily ethane. Natural gas also typically contains varying amounts of a number of contaminants, such as acid gases (CO2 and H2S), nitrogen, helium, water, mercury etc. Before natural gas can be used as a fuel, it must undergo processing to remove almost all material other than methane. Processing usually starts at the wellhead with removal of condensate and water followed by the removal of acid gases by various techniques including amine treating, swing adsorption, etc. Nitrogen may also need to be removed to meet pipeline or final product specifications. Heavier hydrocarbons are typically removed by use of a fractionation train. The fractionation train generally consists of up to three cryogenic distillation towers in series: a deethanizer, depropanizer, and a debutanizer The overhead product from the deethanizer is ethane and the bottoms are fed to the depropanizer. The overhead product from the depropanizer is propane and the bottoms are fed to a debutanizer and the overhead product from debutanizer is typically a mixture of normal and iso-butane, and the bottoms product is a C5+ mixture. The recovered streams of propane and butane and C5+ can be “sweetened” to convert undesirable mercaptans into disulfides and, along with the recovered ethane, are typically the final natural gas liquid (NGL) by-products from the gas processing plant. Alternatively, the NGL stream can be transported directly without fractionation—to a refinery or other processing plant where it can be processed or separated into similar component streams.
Present commercial practices for fractionation and recovery of valuable heavy hydrocarbons (C2+) from natural gas streams involve considerable energy input and equipment. As previously mentioned, in a typical NGL recovery plant the whole gas stream is chilled and passed through multiple distillation columns such as a demethanizer, deethanizer, depropanizer, and debutanizer A purified methane stream from the top of demethanizer tower requires significant recompression to a pressure appropriate for a pipeline. It is estimated that recovery of heavy hydrocarbons from a roughly 1 BSCFD gas stream containing roughly 6% of heavy hydrocarbons requires about 100 MW of power in the NGL plant alone, and an additional about 70 MW for re-compression back to pipeline pressure.
Therefore, there exists a need in the art for the recovery of heavy hydrocarbons from natural gas streams by more economical and efficient processes.